CERN Courier July/August 2016 History Ghosts in the machine Los Alamos National Laboratory National Los Alamos Christine Sutton describes the pioneering 1956 experiment that proved the existence of the neutrino, and how subsequent particle-beam experiments at CERN and elsewhere contributed to unearthing a further two neutrino types.

In July 1956, in a brief paper published in Science, a small team based at the Los Alamos National Laboratory in the US presented results from an experiment at a new, powerful fi ssion reactor at the Savannah River Plant, in South Carolina. The work, they wrote, “verifies the neutrino hypothesis suggested by Pauli”. Clyde Cowan, Fred Reines, Kiko Harrison, Herald Kruse and Austin McGuire had demonstrated for the fi rst time that it was possible to detect neutrinos, setting in motion the new fi eld of neutrino phys- ics. The key ingredients were an intense source and a big detector, with more than a touch of ingenuity and patience. More than two decades previously, in 1930, Wolfgang Pauli had proposed that the “energy crisis” in nuclear beta decay – pre- sented by the continuous energy spectrum of the emitted electron – would be solved if the decaying nucleus also emitted a second, Fred Reines, left, and Clyde Cowan, at the controls of the undetected particle. This would allow the energy released to be Savannah River experiment, which discovered the electron shared between three objects, including the recoiling nucleus, and antineutrino in 1956. so yield electrons with a range of energies, just as observed. The new particle had to be neutral and have a relatively small mass. neutrinos produced during tests of atomic bombs to make a direct Pauli called his proposal “a desperate remedy”, in part because he detection of the elusive particle. He was soon joined in this strange thought that if such a particle did indeed exist, then it “would prob- pursuit by Clyde Cowan, a fellow researcher at Los Alamos, after ably have long ago been seen”. they were stranded together at Kansas Airport, where the conver- Nevertheless, Enrico Fermi took the possibility seriously and sation turned to the “supreme challenge” of detecting neutrinos. based his seminal work on beta decay, published in 1934, on a Reines had an idea to place a detector close to a bomb-test tower point-contact interaction in which a neutron decays to a proton, and use the timing of the detonation as a “gate” to minimise back- electron and (anti)neutrino: n → p e– ν–. Soon afterwards, Hans ground. But what kind of detector? He and Cowan decided on the Bethe and Rudolf Peierls calculated the cross-section for the recently developed medium of liquid scintillator, which could both inverse reaction in which a neutrino is absorbed, but when they act as a target for the inverse beta-decay reaction ν– p → e+ n, and found a value of about 10–44 cm2, the pair concluded that no one detect the emitted positrons via their annihilation to gamma rays. It would be able to detect neutrinos (Bethe and Peierls 1934). What was an audacious plan, not only in taking advantage of a bomb test they did not count on was the discovery of nuclear fi ssion – which but also in scaling up the use of liquid scintillator, which until then on a macroscopic scale produces copious numbers of neutrinos – or had been used only in quantities of about a litre. Reines and Cowan the ingenuity of experimentalists and, later, accelerator physicists. named it “Project Poltergeist”, to refl ect the neutrino’s ghostly nature. Notoriously, nuclear fi ssion was fi rst applied in the atomic bombs Remarkably, the Los Alamos director gave approval for the used towards the end of the Second World War. A few years later, experiment. However, in late 1952, Cowan and Reines were urged in 1951, Fred Reines, a physicist who had worked on the Manhat- to reconsider the more practical idea of using antineutrinos from a ▲ tan Project at Los Alamos, began to think about how to harness the nuclear reactor. The challenge was to work out how to reduce the

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PWAug16Ad_IBIC_full.indd 1 23/06/2016 16:04 CERNCOURIER www. V o l u m e 5 6 N u m b e r 6 J u l y /A u g u s t 2 0 1 6 CERN Courier July/August 2016 CERN Courier July/August 2016 History History Roy Kaltschmidt/LBNL Roy Kaltschmidt/LBNL 1963, thanks to these innovations, CERN had what was at the time l the world’s most intense neutrino beam. ~νe In the 1970s, the combination of the neutrino beam from the PS and Gargamelle – the large bubble chamber built at the Saclay A Laboratory by a team led by André Lagarrigue – led to the discov- II γ γ 3–10 μs ery of weak neutral currents (CERN Courier September 2009 p25), Los AlamosLos National Laboratory γ thereby providing crucial experimental support for the unifi cation

e+ n Cd* of the weak and electromagnetic forces. The neutrino experiments B with Gargamelle also produced key evidence about the existence III p of quarks and, in particular, their fractional charges (CERN Cou- rier April 2014 p24). Then, in 1977, the Super Proton Synchrotron γ γ γ γ (SPS) became the source of neutrino beams at higher energies, and for the next 21 years a series of experiments in CERN’s West Area Fig.2. One of eight detectors at the Daya Bay Reactor Neutrino used neutrinos in experiments covering a broad range of physics, Experiment in China, which are situated within 1.9 km of six Fig.1. (Left) The detector used at Savannah River consisted of three 1400-litre tanks of liquid scintillator (I, II and III), each viewed from neutral currents and the quark structure of matter through nuclear reactors. A larger follow-up experiment called the by 100 phototubes. The smaller tanks (A and B) contained the targets of 200 litres of water doped with cadmium. (Right) The quantum chromodynamics to neutrino oscillations (CERN Cou- Jiangmen Underground Neutrino Observatory (JUNO) is principle of the delayed-coincidence method for detecting the electron antineutrino in the experiment at Savannah River. rier December 1998 p28). currently under development. Around that time, physicists at Fermilab were closing in on a third – backgrounds, because the antineutrino fl ux from a reactor would 1956). After completing many checks, on 14 June 1956 Reines and neutrino type. The DONUT experiment (Direct Observation of antineutrinos (νe) per second and experiments based on the same be thousands of times smaller than that from a nuclear explosion. Cowan sent a jubilant telegram to Pauli in Zurich, informing him the NU Tau) detected neutrinos produced at the Tevatron, and in liquid-scintillator concept continue to provide essential contribu- – Reines and Cowan realised that in addition to looking for positron that they had “defi nitely detected neutrinos from fi ssion fragments 2000, the collaboration announced the discovery of the tau neutrino. tions to neutrino physics by looking for the “disappearance” of the νe. annihilation, they could also detect the neutrons through neutron by observing inverse beta decay of protons”. At the time, Pauli was Although experiments at CERN’s Large Electron–Positron collider Sixty years after the fi rst detection of the neutrino, and more than capture – a process that is delayed for several microseconds, thanks in fact at a meeting at CERN, to where the telegram was forwarded, had already established from precise measurements of the Z boson 80 years after the particle was tentatively predicted, experiments to the neutron’s random walk through a medium prior to inter- and he reportedly interrupted the meeting to read out the good that there are three light neutrino types, the observation of the tau with neutrinos continue to have a leading role in particle physics. acting with a nucleus. In particular, the addition of cadmium to news, later celebrating with a case of champagne (Reines 1979). neutrino completed the leptonic sector of the Standard Model. Today, experimentalists around the world are vying to determine the detector would increase the likelihood of capture and lead to Ten years later, CERN was again setting records for neutrino precisely the mixing parameters of the neutrino, including the the emission of gamma rays. The signature for inverse beta decay The move to accelerators beams, with the CERN Neutrinos to Gran Sasso (CNGS) project, masses. The measurements may prove to hold the answers to some would then be a delayed coincidence between two sets of gamma At the time of the neutrino’s discovery, laboratories such as CERN which directed an intense beam of muon-neutrinos (νμ) to two exper- key questions in the fi eld – ensuring that the “supreme challenge” rays: one from the positron’s annihilation and the other from the and Brookhaven were on their way to building proton synchrotrons iments, ICARUS and OPERA, in the Gran Sasso National Labora- of creating and detecting neutrinos will remain a worthwhile and neutron’s capture. that would have suffi cient energy and intensity to form beams of tory in Italy about 730 km away. CNGS followed the same principle exciting pursuit for the foreseeable future. The detector for Project Poltergeist contained 300 litres of liq- neutrinos via decays of pions and kaons produced when protons as CERN’s early record-breaking beam, this time with protons from uid scintillator with added cadmium chloride, viewed by 90 pho- strike a suitable target. The muons produced in the decays could the SPS. Following fi rst commissioning in 2006 (CERN Courier ● Further reading tomultiplier tubes, and was set up in 1953 at a new reactor at the be stopped by large amounts of shielding, allowing only neutrinos November 2006 p20), the facility ran for physics from 2008 to the H Bethe and R Peierls 1934 Nature 133 532. Hanford Engineering Works in Washington State. This initial to penetrate to experiments beyond. At Brookhaven, this led to end of 2012, and achieved a maximum beam power of 480 kW – the C L Cowan et al. 1956 Science 124 103. experiment showed a small increase in delayed coincidences the discovery at the Alternating Gradient Synchrotron (AGS) in most powerful at the time. A total of 18.24 × 1019 protons were deliv- F Reines and C L Cowan 1953 Phys. Rev. 90 492. when the reactor was operating compared with the situation when 1962 that the neutrinos produced in association with electrons (as ered on target, and the OPERA experiment detected 19,500 neu- F Reines 1979 Science 203 16. it was turned off, but it was set against a cosmic-ray background in beta decay) are different from those produced in association trino events – with fi ve among them identifi ed as a tau neutrino (ντ), that was more than 10 times higher than the expected signal rate with muons (as in pion decay): a second type of neutrino, the muon thereby fi rmly establishing the direct observation of νμ→ ντ oscilla- Résumé (Reines and Cowan 1953). neutrino, had been discovered. tions (CERN Courier July/August 2015 p6). Des fantômes dans la machine This tantalising result encouraged a still more determined effort, In 1963, an ingenious way to produce neutrino beams of greater with a new detector design that was basically a sandwich with three intensity fi rst came into use at the Proton Synchrotron (PS) at A bountiful legacy En 1956, Frederick Reines et Clyde Cowan découvrirent le layers of liquid scintillator and two layers of water with added cad- CERN, where Simon van der Meer had described his concept of Since the fi rst glimpses of antineutrino interactions 60 years ago in neutrino en menant une expérience auprès d’un réacteur sur mium chloride to act as the target (fi gure 1). Positrons produced in the neutrino horn a couple of years earlier (van der Meer 1961). reactor experiments, experiments have gone on to detect neutrinos le site de Savannah River (États-Unis), plus de 25 ans après la a neutrino interaction would be detected almost immediately via Because neutrinos are electrically neutral, they cannot be focused and antineutrinos produced in a variety of ways – both in beams première prédiction de son existence. Il s’agissait d’une expérience two back-to-back gamma rays in the adjacent scintillator tanks, into a beam using magnets, so he devised instead a way to focus created at particle accelerators and also naturally by reactions in pionnière, dont les principes sont utilisés encore aujourd’hui pour which would be followed a few microseconds later by another burst the parent pions and kaons using magnetic fi elds set up by cur- the Sun, interactions of cosmic rays in the Earth’s atmosphere and, les expériences neutrino modernes auprès de réacteurs. Après of gamma rays in the same two rents circulating in a metallic cone-shaped “horn” (CERN Courier most recently, astrophysical processes. We now know that neutri- cette découverte, les physiciens ont commencé à chercher d’autres

scintillator tanks, this time from June 2011 p24). The device concentrated neutrinos produced as nos exist not only in three fl avour eigenstates – electron ν( e), muon manières d’étudier les neutrinos, notamment en utilisant des ν ν ν ν On 14 June 1956, neutron capture. the charged particles decayed in fl ight into a beam, and because ( μ) and tau ( τ) – but also in different mass eigenstates ( 1, 2 and faisceaux de particules créés à cet effet, développés d’abord au CERN The second experiment ran it could focus either positive or negative particles, it produced ν3) with very small masses, and that they can oscillate from one et à Brookhaven (États-Unis). Cette méthode a permis de découvrir Reines and Cowan at the newly completed Savan- an almost pure beam of neutrinos (from positive parents) or fl avour to another through quantum-mechanical mixing (see p34). le neutrino muonique et le neutrino tauique, respectivement en 1962 sent a jubilant nah River Plant for a total of antineutrinos (negative parents). A second technical innovation at Reactor experiments – in particular Double Chooz in France, et en 2000. Aujourd’hui, divers autres types d’expériences neutrino 1371 hours in 1956 and, when CERN enabled the horn to become a formidable device: the tech- the Daya Bay Reactor Neutrino Experiment in China (fi gure 2) and sondent les propriétés de ces particules si insaisissables. telegram to Pauli the reactor was on, it recorded nique of “fast ejection”, devised by Berend Kuiper and Günther the Reactor Experiment for Neutrino Oscillation (RENO) in South in Zurich. nearly three delayed coinci- Plass, could direct all of the protons from one cycle of the PS onto Korea – are still as relevant now as they were in Cowan and Reines’ Christine Sutton, former editor of the CERN Courier and author of Spaceship dences per hour (Cowan et al. the target at the mouth of the horn (Kuiper and Plass 1959). By mid- day. Modern nuclear power plants produce about 1020 electron Neutrino (CUP 1992).

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CERNCOURIER www. V o l u m e 5 6 N u m b e r 6 J u l y /A u g u s t 2 0 1 6 CERN Courier July/August 2016 CERN Courier July/August 2016 History History Roy Kaltschmidt/LBNL Roy Kaltschmidt/LBNL 1963, thanks to these innovations, CERN had what was at the time l the world’s most intense neutrino beam. ~νe In the 1970s, the combination of the neutrino beam from the PS and Gargamelle – the large bubble chamber built at the Saclay A Laboratory by a team led by André Lagarrigue – led to the discov- II γ γ 3–10 μs ery of weak neutral currents (CERN Courier September 2009 p25), Los AlamosLos National Laboratory γ thereby providing crucial experimental support for the unifi cation

e+ n Cd* of the weak and electromagnetic forces. The neutrino experiments B with Gargamelle also produced key evidence about the existence III p of quarks and, in particular, their fractional charges (CERN Cou- rier April 2014 p24). Then, in 1977, the Super Proton Synchrotron γ γ γ γ (SPS) became the source of neutrino beams at higher energies, and for the next 21 years a series of experiments in CERN’s West Area Fig.2. One of eight detectors at the Daya Bay Reactor Neutrino used neutrinos in experiments covering a broad range of physics, Experiment in China, which are situated within 1.9 km of six Fig.1. (Left) The detector used at Savannah River consisted of three 1400-litre tanks of liquid scintillator (I, II and III), each viewed from neutral currents and the quark structure of matter through nuclear reactors. A larger follow-up experiment called the by 100 phototubes. The smaller tanks (A and B) contained the targets of 200 litres of water doped with cadmium. (Right) The quantum chromodynamics to neutrino oscillations (CERN Cou- Jiangmen Underground Neutrino Observatory (JUNO) is principle of the delayed-coincidence method for detecting the electron antineutrino in the experiment at Savannah River. rier December 1998 p28). currently under development. Around that time, physicists at Fermilab were closing in on a third – backgrounds, because the antineutrino fl ux from a reactor would 1956). After completing many checks, on 14 June 1956 Reines and neutrino type. The DONUT experiment (Direct Observation of antineutrinos (νe) per second and experiments based on the same be thousands of times smaller than that from a nuclear explosion. Cowan sent a jubilant telegram to Pauli in Zurich, informing him the NU Tau) detected neutrinos produced at the Tevatron, and in liquid-scintillator concept continue to provide essential contribu- – Reines and Cowan realised that in addition to looking for positron that they had “defi nitely detected neutrinos from fi ssion fragments 2000, the collaboration announced the discovery of the tau neutrino. tions to neutrino physics by looking for the “disappearance” of the νe. annihilation, they could also detect the neutrons through neutron by observing inverse beta decay of protons”. At the time, Pauli was Although experiments at CERN’s Large Electron–Positron collider Sixty years after the fi rst detection of the neutrino, and more than capture – a process that is delayed for several microseconds, thanks in fact at a meeting at CERN, to where the telegram was forwarded, had already established from precise measurements of the Z boson 80 years after the particle was tentatively predicted, experiments to the neutron’s random walk through a medium prior to inter- and he reportedly interrupted the meeting to read out the good that there are three light neutrino types, the observation of the tau with neutrinos continue to have a leading role in particle physics. acting with a nucleus. In particular, the addition of cadmium to news, later celebrating with a case of champagne (Reines 1979). neutrino completed the leptonic sector of the Standard Model. Today, experimentalists around the world are vying to determine the detector would increase the likelihood of capture and lead to Ten years later, CERN was again setting records for neutrino precisely the mixing parameters of the neutrino, including the the emission of gamma rays. The signature for inverse beta decay The move to accelerators beams, with the CERN Neutrinos to Gran Sasso (CNGS) project, masses. The measurements may prove to hold the answers to some would then be a delayed coincidence between two sets of gamma At the time of the neutrino’s discovery, laboratories such as CERN which directed an intense beam of muon-neutrinos (νμ) to two exper- key questions in the fi eld – ensuring that the “supreme challenge” rays: one from the positron’s annihilation and the other from the and Brookhaven were on their way to building proton synchrotrons iments, ICARUS and OPERA, in the Gran Sasso National Labora- of creating and detecting neutrinos will remain a worthwhile and neutron’s capture. that would have suffi cient energy and intensity to form beams of tory in Italy about 730 km away. CNGS followed the same principle exciting pursuit for the foreseeable future. The detector for Project Poltergeist contained 300 litres of liq- neutrinos via decays of pions and kaons produced when protons as CERN’s early record-breaking beam, this time with protons from uid scintillator with added cadmium chloride, viewed by 90 pho- strike a suitable target. The muons produced in the decays could the SPS. Following fi rst commissioning in 2006 (CERN Courier ● Further reading tomultiplier tubes, and was set up in 1953 at a new reactor at the be stopped by large amounts of shielding, allowing only neutrinos November 2006 p20), the facility ran for physics from 2008 to the H Bethe and R Peierls 1934 Nature 133 532. Hanford Engineering Works in Washington State. This initial to penetrate to experiments beyond. At Brookhaven, this led to end of 2012, and achieved a maximum beam power of 480 kW – the C L Cowan et al. 1956 Science 124 103. experiment showed a small increase in delayed coincidences the discovery at the Alternating Gradient Synchrotron (AGS) in most powerful at the time. A total of 18.24 × 1019 protons were deliv- F Reines and C L Cowan 1953 Phys. Rev. 90 492. when the reactor was operating compared with the situation when 1962 that the neutrinos produced in association with electrons (as ered on target, and the OPERA experiment detected 19,500 neu- F Reines 1979 Science 203 16. it was turned off, but it was set against a cosmic-ray background in beta decay) are different from those produced in association trino events – with fi ve among them identifi ed as a tau neutrino (ντ), that was more than 10 times higher than the expected signal rate with muons (as in pion decay): a second type of neutrino, the muon thereby fi rmly establishing the direct observation of νμ→ ντ oscilla- Résumé (Reines and Cowan 1953). neutrino, had been discovered. tions (CERN Courier July/August 2015 p6). Des fantômes dans la machine This tantalising result encouraged a still more determined effort, In 1963, an ingenious way to produce neutrino beams of greater with a new detector design that was basically a sandwich with three intensity fi rst came into use at the Proton Synchrotron (PS) at A bountiful legacy En 1956, Frederick Reines et Clyde Cowan découvrirent le layers of liquid scintillator and two layers of water with added cad- CERN, where Simon van der Meer had described his concept of Since the fi rst glimpses of antineutrino interactions 60 years ago in neutrino en menant une expérience auprès d’un réacteur sur mium chloride to act as the target (fi gure 1). Positrons produced in the neutrino horn a couple of years earlier (van der Meer 1961). reactor experiments, experiments have gone on to detect neutrinos le site de Savannah River (États-Unis), plus de 25 ans après la a neutrino interaction would be detected almost immediately via Because neutrinos are electrically neutral, they cannot be focused and antineutrinos produced in a variety of ways – both in beams première prédiction de son existence. Il s’agissait d’une expérience two back-to-back gamma rays in the adjacent scintillator tanks, into a beam using magnets, so he devised instead a way to focus created at particle accelerators and also naturally by reactions in pionnière, dont les principes sont utilisés encore aujourd’hui pour which would be followed a few microseconds later by another burst the parent pions and kaons using magnetic fi elds set up by cur- the Sun, interactions of cosmic rays in the Earth’s atmosphere and, les expériences neutrino modernes auprès de réacteurs. Après of gamma rays in the same two rents circulating in a metallic cone-shaped “horn” (CERN Courier most recently, astrophysical processes. We now know that neutri- cette découverte, les physiciens ont commencé à chercher d’autres

scintillator tanks, this time from June 2011 p24). The device concentrated neutrinos produced as nos exist not only in three fl avour eigenstates – electron ν( e), muon manières d’étudier les neutrinos, notamment en utilisant des ν ν ν ν On 14 June 1956, neutron capture. the charged particles decayed in fl ight into a beam, and because ( μ) and tau ( τ) – but also in different mass eigenstates ( 1, 2 and faisceaux de particules créés à cet effet, développés d’abord au CERN The second experiment ran it could focus either positive or negative particles, it produced ν3) with very small masses, and that they can oscillate from one et à Brookhaven (États-Unis). Cette méthode a permis de découvrir Reines and Cowan at the newly completed Savan- an almost pure beam of neutrinos (from positive parents) or fl avour to another through quantum-mechanical mixing (see p34). le neutrino muonique et le neutrino tauique, respectivement en 1962 sent a jubilant nah River Plant for a total of antineutrinos (negative parents). A second technical innovation at Reactor experiments – in particular Double Chooz in France, et en 2000. Aujourd’hui, divers autres types d’expériences neutrino 1371 hours in 1956 and, when CERN enabled the horn to become a formidable device: the tech- the Daya Bay Reactor Neutrino Experiment in China (fi gure 2) and sondent les propriétés de ces particules si insaisissables. telegram to Pauli the reactor was on, it recorded nique of “fast ejection”, devised by Berend Kuiper and Günther the Reactor Experiment for Neutrino Oscillation (RENO) in South in Zurich. nearly three delayed coinci- Plass, could direct all of the protons from one cycle of the PS onto Korea – are still as relevant now as they were in Cowan and Reines’ Christine Sutton, former editor of the CERN Courier and author of Spaceship dences per hour (Cowan et al. the target at the mouth of the horn (Kuiper and Plass 1959). By mid- day. Modern nuclear power plants produce about 1020 electron Neutrino (CUP 1992).

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When CERN was founded in 1954, the neutrino was technically were eventually established by the Super-Kamiokande collaboration still a fi gment of theorists’ imaginations. Six decades later, neu- in Japan and the Sudbury Neutrino Observatory in Canada. More trinos have become the most studied of all elementary particles. recently, from 2006 to 2012, CERN sent a muon-neutrino beam Several new and upgraded neutrino-beam experiments planned in to the ICARUS and OPERA detectors at the Gran Sasso National Japan and the US, in addition to the reactor-based JUNO experi- Laboratory, 732 km away in Italy. The main goal was to observe ment in China, aim to measure vital parameters such as the order- the transformation of muon neutrinos into tau neutrinos, which was ing of the neutrino masses and potential CP-violating effects in confi rmed by the OPERA collaboration in 2015. the neutrino sector. In support of this effort, CERN is mounting a Following the recommendations of the European Strategy for signifi cant R&D programme called the CERN Neutrino Platform Particle Physics in 2013, CERN inaugurated the neutrino plat- to strengthen European participation in neutrino physics. form at the end of 2014. Its aim is to provide a focal point for CERN has a long tradition in neutrino physics. It was the study Europe’s contributions to global neutrino research by developing of neutrino beams with the Gargamelle detector at CERN in 1973 and prototyping the next generation of neutrino detectors. So far, AC/DC DC/DC SYSTEMS SOLUTIONS CONTROL that provided the fi rst evidence for the weak neutral current, and in around 50 European institutes have signed up as members of the the late 1970s, three experiments – BEBC, CDHS and CHARM neutrino platform, which sees CERN shift from its traditional role – used a beam from the SPS to further unveil the neutrino’s iden- of providing neutrino beams to one where it shares its expertise in tity. A milestone came in 1989, when precise measurements at the detectors, infrastructure and international collaboration. Control almost every iseg HV supply. Info and live test: Large Electron–Positron Collider showed that there are three, and “The neutrino platform pulls together a community that is scat- 35 YEARS only three, types of light neutrinos that couple to the Z boson. This tered across the world and CERN has committed significant was followed by searches for neutrino oscillations at NOMAD (also resources to support R&D in all aspects of neutrino research,”

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